As is well known, carbon artifacts have been made by pyrolyzing a wide variety of organic materials. Indeed, one carbon artifact of particularly important commercial interest today is carbon fiber. Hence, specific reference is made herein to carbon fiber technology. Nevertheless, it should be appreciated that this invention has applicability to carbon artifact manufacturing generally, and most particularly, to the production of shaped carbon articles in the form of filaments, yarns, films, ribbons, sheets and the like.
Referring now in particular to carbon fibers, suffice it to say, that the use of carbon fibers in reinforcing plastic and metal matrices has gained considerable commercial acceptance where the exceptional properties of the reinforcing composite materials, such as their higher strength to weight ratio, clearly offset the generally higher costs associated with preparing them. It is generally accepted that large scale use of carbon fibers as a reinforcing material would gain even greater acceptance in the marketplace if the costs associated with the formation of the fibers could be substantially reduced. Thus, formation of carbon fibers for relatively inexpensive carbonaceous pitches has received considerable attention in recent years.
Many carbonaceous pitches are known to be converted at the early stage of carbonization to a structurally ordered optically anisotropic spherical liquid crystal called mesophase. The presence of this ordered structure prior to carbonization is considered to be a significant determinant of the fundamental properties of a carbon artifact made from such a carbonaceous pitch. Indeed, the ability to generate high optical anisotropicity during processing is accepted, particularly in carbon fiber production, as a prerequisite for the formation of high quality products. Thus, one of the first requirements of a feedstock material suitable for carbon artifact manufacture, and particularly for carbon fiber production, is its ability to be converted to a highly optically anisotropic material.
In addition to being able to develop highly ordered structures, suitable feedstocks for carbon artifacts manufacture, and in particular carbon fiber manufacture, should have relatively low softening points rendering them suitable for being deformed and shaped into desirable articles. Thus, in carbon fiber manufacture a suitable pitch which is capable of generating the requisite highly ordered structure also must exhibit sufficient viscosity for spinning. Unfortunately, many carbonaceous pitches have relatively high softening points. Indeed, incipient coking frequently occurs in such materials at temperatures where they have sufficient viscosity for spinning. The presence of coke, however, or other infusible materials and/or undesirable high softening point components generated prior to or at the spinning temperatures are detrimental to fiber processability and are believed to be detrimental to fiber product quality.
As is well known, pitches have been prepared from the residues and tars obtained from steam cracking of gas oil or naphtha. In this regard, see, for example, U.S. Pat. Nos. 3,721,658 and 4,086,156. These tarry products typically are composed of alkyl substituted polynuclear aromatics. Indeed, steam cracker tars have relatively high levels of paraffinic carbon atoms, for example, in the range of about 30 atom % to about 35 atom % paraffinic carbon atoms, the presence of which tends to be detrimental on the formation of a suitable anisotropic pitch for carbon fiber production. Additionally, steam cracker tars contain asphaltenes in relatively large quantities, for example, in the range of about 20 to about 30 weight percent. Asphaltenes, as is well known, are solids which are insoluble at paraffinic solvents. The asphaltenes on carbonization tend to form isotropic material, rather than anisotropic material, and hence its presence in steam cracker tars tends to be detrimental in the formation of anisotropic pitch from such steam cracker tars.
As mentioned above, many isotropic carbonaceous pitch materials can be converted to an optically anisotropic phase by thermal treatment of the isotropic material. In the instance of steam cracker tars, however, thermal heat treatment of the steam cracker tars provides an isotropic pitch containing components which have a softening point which is undesirably high, for example, greater than 375.degree. C., for carbon artifact manufacture, particularly for carbon filter manufacture. In other words, the thermal generation of pitches from steam cracker tars has not, heretofore, been known in the art to form pitches having high optical anisotropicity, e.g., greater than 70%, and low softening points and viscosities, e.g., below about 325.degree. C. and 2000 poise (at 360.degree. C.).